Messages in this thread Patch in this message | | | Subject | Re: Q: perf_event && event->owner | From | Peter Zijlstra <> | Date | Fri, 12 Nov 2010 16:48:03 +0100 |
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On Wed, 2010-11-10 at 16:44 +0100, Oleg Nesterov wrote: > > But the code is racy, yes. owner != NULL case is fine. But > perf_release() can see event->owner == NULL before list_del() was > completed. perf_event_exit_task() needs wmb() in between, I think. >
Utter paranoia took over and I'm still not sure its solid...
--- Subject: perf: Fix owner-list vs exit From: Peter Zijlstra <a.p.zijlstra@chello.nl> Date: Tue, 09 Nov 2010 19:01:43 +0100
Reported-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> LKML-Reference: <1289325703.2191.60.camel@laptop> --- kernel/perf_event.c | 63 ++++++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 51 insertions(+), 12 deletions(-)
Index: linux-2.6/kernel/perf_event.c =================================================================== --- linux-2.6.orig/kernel/perf_event.c +++ linux-2.6/kernel/perf_event.c @@ -2234,11 +2234,6 @@ int perf_event_release_kernel(struct per raw_spin_unlock_irq(&ctx->lock); mutex_unlock(&ctx->mutex); - mutex_lock(&event->owner->perf_event_mutex); - list_del_init(&event->owner_entry); - mutex_unlock(&event->owner->perf_event_mutex); - put_task_struct(event->owner); - free_event(event); return 0; @@ -2251,9 +2246,43 @@ EXPORT_SYMBOL_GPL(perf_event_release_ker static int perf_release(struct inode *inode, struct file *file) { struct perf_event *event = file->private_data; + struct task_struct *owner; file->private_data = NULL; + rcu_read_lock(); + owner = ACCESS_ONCE(event->owner); + /* + * Matches the smp_wmb() in perf_event_exit_task(). If we observe + * !owner it means the list deletion is complete and we can indeed + * free this event, otherwise we need to serialize on + * owner->perf_event_mutex. + */ + smp_read_barrier_depends(); + if (owner) { + /* + * Since delayed_put_task_struct() also drops the last + * task reference we can safely take a new reference + * while holding the rcu_read_lock(). + */ + get_task_struct(owner); + } + rcu_read_unlock(); + + if (owner) { + mutex_lock(&owner->perf_event_mutex); + /* + * We have to re-check the event->owner field, if it is cleared + * we raced with perf_event_exit_task(), acquiring the mutex + * ensured they're done, and we can proceed with freeing the + * event. + */ + if (event->owner) + list_del_init(&event->owner_entry); + mutex_unlock(&owner->perf_event_mutex); + put_task_struct(owner); + } + return perf_event_release_kernel(event); } @@ -5677,7 +5706,7 @@ SYSCALL_DEFINE5(perf_event_open, mutex_unlock(&ctx->mutex); event->owner = current; - get_task_struct(current); + mutex_lock(¤t->perf_event_mutex); list_add_tail(&event->owner_entry, ¤t->perf_event_list); mutex_unlock(¤t->perf_event_mutex); @@ -5745,12 +5774,6 @@ perf_event_create_kernel_counter(struct ++ctx->generation; mutex_unlock(&ctx->mutex); - event->owner = current; - get_task_struct(current); - mutex_lock(¤t->perf_event_mutex); - list_add_tail(&event->owner_entry, ¤t->perf_event_list); - mutex_unlock(¤t->perf_event_mutex); - return event; err_free: @@ -5901,8 +5924,24 @@ static void perf_event_exit_task_context */ void perf_event_exit_task(struct task_struct *child) { + struct perf_event *event, *tmp; int ctxn; + mutex_lock(&child->perf_event_mutex); + list_for_each_entry_safe(event, tmp, &child->perf_event_list, + owner_entry) { + list_del_init(&event->owner_entry); + + /* + * Ensure the list deletion is visible before we clear + * the owner, closes a race against perf_release() where + * we need to serialize on the owner->perf_event_mutex. + */ + smp_wmb(); + event->owner = NULL; + } + mutex_unlock(&child->perf_event_mutex); + for_each_task_context_nr(ctxn) perf_event_exit_task_context(child, ctxn); }
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